1. Field of the Invention
The present invention relates to a passive entry having an anti-theft function, and more particularly to a passive entry having an anti-theft function for preventing damage of relay attack by use of plural repeaters so that elapsed time from transmission of a request signal to a car-mounted transmitter-receiver to reception of an answer signal is counted, and only when the counted time value is within a valid elapsed-time value, a car door is locked or unlocked.
2. Description of the Prior Art
Generally, a passive entry used with a car comprises a transmitter-receiver mounted in the car and one or more portable transmitter-receivers for performing bidirectional signal transmission with the car-mounted transmitter-receiver. In the passive entry, during bidirectional signal transmission, the car-mounted transmitter-receiver transmits a request signal by a low frequency carrier signal, and if, at a transmission timing of the request signal, the portable transmitter-receivers exist in the proximity of the car, that is, within a reach range of the low frequency carrier signal, the portable transmitter-receivers receive the request signal. At this time, the portable transmitter-receivers transmit an answer signal by a high frequency carrier signal in response to the received request signal, and when the car-mounted transmitter-receiver receives the answer signal, an apparatus mounted in the car, e.g., a door is automatically locked or unlocked by the car-mounted transmitter-receiver. That is, upon receiving a legal answer signal when the door is locked, the car-mounted transmitter-receiver automatically unlocks the door, and on the other hand, upon receiving a legal answer signal when the door is unlocked, it automatically locks the door.
According to the passive entry having such a function, when a person carrying a portable transmitter-receiver approaches a car, a car door can be automatically unlocked without having to operate the portable transmitter-receiver; on the other hand, when the person carrying the portable transmitter-receiver moves away from the car, the car door can be automatically locked without having to operate the portable transmitter-receiver.
In the existing passive entry, when unlocking the car door, the person carrying the portable transmitter-receiver has only to approach the car, and on the other hand, when locking the car door, the person carrying the portable transmitter-receiver has only to move away from the car. Therefore, the person takes no trouble to lock or unlock the car door by a key in hand or by operating the operation keys of the portable transmitter-receiver.
As described above, in the existing passive entry, when unlocking the car door, the person carrying the portable transmitter-receiver must approach the car. However, if repeaters are used to relay a low frequency carrier signal for transmitting a request signal and a high frequency carrier signal for transmitting an answer signal, the car door can be unlocked even if the person carrying the portable transmitter-receiver does not approach the car.
FIG. 5 is a diagram for explaining a state in which bidirectional communication is possible between a car-mounted transmitter-receiver and a portable transmitter-receiver if two repeaters are used when the portable transmitter-receiver is distant from a car.
In the example shown in FIG. 5, since the owner (not provided with a reference numeral) of a car 51 having a portable transmitter-receiver 52 is considerably distant from the car 51, the portable transmitter-receiver 52 is placed outside a reach range EL of a request signal from a car-mounted transmitter-receiver (not provided with a reference numeral) mounted on the car 51. On the other hand, since the car-mounted transmitter-receiver is placed outside a reach range of an answer signal from the portable transmitter-receiver 52, communication between the car-mounted transmitter-receiver and the portable transmitter-receiver 52 is impossible in this state.
In the example shown in FIG. 5, a first repeater 531 and a second repeater 532 intervene between the car-mounted transmitter-receiver (not provided with a reference numeral) mounted in the car 51 and the portable transmitter-receiver 52 distant from the car so that a request signal and an answer signal can be transmitted between the car-mounted transmitter-receiver and the portable transmitter-receiver 52. The first repeater 531 is placed within a reach range EL of a low frequency carrier signal (request signal) transmitted by the car-mounted transmitter-receiver and the second relay 532 is placed within a reach range EL of a low frequency carrier signal (request signal) transmitted by the portable transmitter-receiver 52. Upon receiving the low frequency carrier signal (request signal) from the car-mounted transmitter-receiver, the first repeater 531 converts the low frequency carrier signal to a high frequency carrier signal to transmit the request signal to the second repeater 532. Upon receiving the request signal transmitted by the high frequency carrier signal, the second repeater 532 converts the high frequency carrier signal to a low frequency carrier signal and transmits the request signal to the portable transmitter-receiver 52 by the low frequency carrier signal so that the request signal is transmitted to the portable transmitter-receiver 52. On the other hand, upon receiving a high frequency carrier signal (answer signal) from the portable transmitter-receiver 52, the second repeater 532 amplifies the received high frequency carrier signal and then transmits the answer signal to the first repeater 531 by the high frequency carrier signal. Upon receiving the answer signal transmitted by the high frequency carrier signal, the first repeater 531 amplifies the received high frequency carrier signal and then transmits the answer signal to the car-mounted transmitter-receiver by the high frequency carrier signal so that the answer signal is transmitted to the car-mounted transmitter-receiver.
In the existing passive entry, as shown in FIG. 5, if the repeaters 531 and 532 are used, since the reach range EL of the low frequency carrier signal (request signal), and in some cases, a reach range of the high frequency carrier signal (answer signal) can be substantially expanded, the owner of the car 51 carrying the portable transmitter-receiver 52 can unlock a car door even if he is considerably distant from the car 51. The above-described act is used to unlock a car door by malicious people without permission of a car owner and is called relay attack.
The existing passive entry has the risk of a car door being unlocked using the repeaters 531 and 532 by malicious people, that is, undergoing relay attack.